1. Field of the Invention
The invention relates to a carrier recovery system for use in illustratively a passband QAM (quadrature amplitude modulation) demodulator and specifically such a system, including apparatus and accompanying methods, that employs separate acquisition and tracking modes and automatic carrier-to-noise estimation.
2. Description of the Prior Art
Quadrature amplitude modulation (QAM) is increasingly seeing use as an attractive vehicle to transmit digital data. In that regard, several proposals exist in the art to broadcast high definition television (HDTV) signals as compressed digitized data using QAM.
In essence, QAM relies on transmitting data as a sequence of two-dimensional complex symbols, i.e. with both in-phase and quadrature components. Each symbol, based upon the data it represents, takes on a specific pre-defined value. A set of all of the values available for transmission defines an alphabet which, when graphically plotted, typically on a two-dimensional basis, forms a constellation. The size and shape of the constellation depends upon the number of discrete values in the set and their spatial location in the constellation. The constellation frequently proposed for use in broadcasting HDTV data contains, e.g., 16, 32 or 64 values (states), hence so-called 16, 32 or 64 QAM, respectively.
To receive broadcast QAM data, a QAM receiver essentially samples and filters a received output of a communication channel, and applies resulting filtered samples to a decoder (e.g. a Viterbi decoder), which contains one or more slicers, to yield detected symbols. The data contained in these latter symbols, if it contains compressed video information, is then appropriately decompressed to yield original source video data. To specifically accomplish QAM reception, a QAM demodulator within the receiver performs the functions of timing recovery, equalization and carrier recovery. To the extent relevant here, timing recovery, which typically occurs at one, two and/or four times a rate at which symbols are received, defines precise instances in time at which a received data stream must be sampled in order to minimize inter-symbol interference and establish a timing baseline at which a decision is to be made for each received symbol. In essence, timing recovery relies on accurately recovering, at a proper frequency, a clock signal from a received modulated waveform. This clock signal is used, in turn, to convert a continuous-time received signal into a discrete-time sequence of symbols. Equalization is essentially a filtering function which, generally adaptive, removes channel-induced artifacts and reduces inter-symbol interference (interference caused by amplitude and phase dispersion of the transmitted symbols that results from passage through the channel) from the received symbols. Carrier recovery, typically performed on a decision-directed basis and in the usual absence of a pilot tone, creates a reference carrier against which in-phase and quadrature modulated components may be determined, e.g. both in terms of frequency and phase, such that the received demodulated symbols do not rotate. It is the carrier signal that is quadrature modulated by the symbols and then transmitted to the receiver. Carrier recovery must be able to properly function in the presence of varying frequency offsets, drift or jitter that often occurs between a QAM transmitter and the receiver. Through carrier recovery, a carrier frequency offset value is translated into a value, typically a direct current (DC) or digitized value, that is generally and respectively fed as a control input to a voltage or numerically controlled oscillator situated within a phase-locked loop. The output of this oscillator, being locked in frequency and phase to the reference carrier signal, is then applied as, e.g., a local oscillator in a tuner section of the receiver to extract, e.g., baseband quadrature modulated, information from the continuous-time received signal. Inasmuch as the present invention is directed to carrier recovery systems, the remainder of the discussion will be so limited.
In general, carrier recovery is performed directly after equalization. Simplistically and conventionally speaking, carrier recovery involves applying the received symbols as one input to a de-rotator, specifically a complex multiplier. Quadrature (sine/cosine) outputs of a numerically controlled oscillator (NCO) are applied to another input of the de-rotator. To assure that the quadrature outputs of the oscillator are locked to and accurately track the carrier., in terms of both frequency and phase, essentially regardless of jitter in the carrier or frequency and/or phase offsets in the carrier between the transmitter and receiver, this oscillator is situated within a digital phase-locked loop (DPLL). This loop contains a phase comparator which determines, on a decision-directed basis, the phase error of a received constellation by comparing the phase of the output of the de-rotator to assumed ideal symbol positions in the same constellation. This determination entails comparing the phase error between each de-rotated received symbol and a resulting sliced symbol therefor. The resulting phase error signal is applied through a loop filter and then supplied to the NCO, which itself comprises an integrator (phase accumulator) and a sine/cosine look-up table. The de-rotated symbols are provided, as the detected symbols, to an output of the demodulator for subsequent decoding and, where suitable, decompression.
Conventional carrier recovery systems, of the type described above, suffer various drawbacks which tend to limit their utility, particularly, though not exclusively, when used in demodulating HDTV data.
First, these systems rely on determining the phase error by comparing the full constellation of the received symbols against their corresponding ideal values. Unfortunately, in practice, this approach often fails to achieve a lock or, should a lock occur, often leads to a false lock--the latter being a lock at certain erroneous phase shifts at which the resulting de-rotated constellation remains stable--in essence the de-rotated constellation remains tilted from its ideal orientation. These erroneous phase shifts are defined by the position of intermediate "zeroes" in the average phase error produced by the DPLL in this carrier recovery system. In such instances, each de-rotated received symbol typically falls within a corresponding decision region but for the wrong corresponding detected symbol. Consequently, the resulting detected symbols would simply be wrong and totally unusable. Nevertheless, since a resulting phase error, when averaged over many such received symbols, tends to remain close to zero, a conventional carrier recovery system will simply maintain the false lock.
Second, a certain number of symbols and hence time are both needed to acquire (or re-acquire) a phase lock. While ordinarily, for an HDTV transmission, the amount of symbols and attendant data loss is negligible, in certain instances the data loss can be quite objectionable to a viewer.
In particular, it is widely recognized in the art that, prior to the point at which the carrier recovery circuit is able to acquire a proper phase lock on the received symbols (particularly when accompanied by a phase error), this circuit will make wrong decisions and the received constellation will, in essence, continue to rotate. Eventually, the lock will be achieved and the received constellation will cease rotating though many symbols, e.g., tens of thousands (or more), may often need to occur in order to draw the circuit into lock. The number of erroneous decisions will decrease only after the lock occurs. Since the symbols used in achieving the lock are generally erroneous, the data content of all of these symbols is simply ignored. What this means is simply that the data, transmitted while a QAM receiver is attaining a carrier recovery phase lock, is simply lost to any downstream circuitry connected to the receiver. This, in turn, in the context of a received HDTV transmission, means that any accompanying broadcast visual and/or aural information or data occurring while a phase lock is being acquired, is not provided to the viewer.
At expected HDTV data rates of approximately 20-25 Mbits/second, a phase lock can often occur within a relatively short time, such as on the order of approximately a few milliseconds or so. Ordinarily, in reception areas that possess strong, stable received signals, the lock will generally persist for quite an extended period of time. Hence, the amount of data lost to a viewer while re-acquiring a phase lock as a function of the total amount of transmitted data will usually be quite small and thus generally imperceptible. However, in areas with varying and particularly poor signal strength, a phase lock may exist over only a relatively short interval thereby necessitating repeated re-acquisition of the lock. Consequently, the amount of data that is lost, as a percentage of the total transmitted data, could sharply increase. Hence, if the re-acquisition occurs sufficiently often, the relatively large amount of lost data can result in a highly objectionable image to a viewer.
Furthermore, larger constellations advantageously permit each symbol to contain a substantially increased data content, thereby, e.g., providing increasingly fine image resolution in a displayed HDTV image. Generally, an increased constellation, e.g. 32 or 64 state for HDTV, might be used in strong signal areas, with the constellation size being reduced by an HDTV broadcaster to 16 or 32 state, respectively, wherever interference is likely (such as, e.g., from a conventional "NTSC" television signal appearing on the same channel and transmitted from a neighboring geographic area). However, increasingly large constellations provide a correspondingly decreased slicing decision region around each symbol. This, in turn, aggravates the effect of the phase errors associated with each symbol. Consequently, an increased number of symbols (and hence time and delay) is required to achieve the phase lock. Therefore, where a larger constellation is used, a correspondingly increased amount of data must be expended and lost to the viewer each time a carrier recovery phase lock must be re-acquired. Should the received signal deteriorate, such as in the presence of noise or interference, repeated re-acquisition of the phase lock particularly for a large constellation, can erode image quality more rapidly than if a small constellation were to be broadcast.
Given these deficiencies, the art teaches that for a carrier recovery circuit to rapidly and accurately achieve a phase lock, particularly in hose instances where the lock could not be acquired by slicing using a full constellation, reduced constellation (RC) slicing should be used instead. In this regard, see N. K. Jablon, "Joint Blind Equalization, Carrier Recovery, and Timing Recovery for High-Order QAM Signal Constellations", IEEE Transactions on Signal Processing, Vol. 40, No. 6, June 1992, pages 1383-1397; and N. K. Jablon, "Joint Blind Equalization, Carrier Recovery, and Timing Recovery for 64-QAM and 128-QAM Signal Constellations", Record of IEEE International Conference on Communications (Boston, Mass.), Jun. 11-14, 1989, pages 1043-1049 (both of which are collectively hereinafter referred to as the "Jablon" publications). As described in the Jablon publications, for 16- and 64-QAM, a phase lock is first acquired through a phase comparison whenever just a pre-defined one of four corner symbols in the constellation, rather than all the symbols in the constellation, is detected. This particular corner symbol is detected by comparing the squared magnitude of all the received symbols against a pre-defined threshold. If the squared magnitude of a received symbol equals or exceeds the threshold, then the phase comparison occurs between this symbol and its ideal value. Otherwise, if the received symbol is less than the threshold, the output of the phase comparator is set to zero. However, the loop filter is updated normally every symbol period in order to permit the DPLL to track any frequency offset. Jablon postulates that since all the constellation symbols are equally affected by additive noise and adaptation noise, all the corner symbols in the constellation, which all have a longest radii and thus a largest signal to additive-plus-adaptive noise ratio of all the symbols, provide the most reliable information regarding the current orientation of the constellation. Throughout an interval defined by the occurrence of a finite number of symbols (a finite time interval) and during which reduced constellation slicing is used, the constellation will presumably become aligned and a phase lock achieved, i.e. acquired. Once this lock occurs, the carrier is then tracked, on a decision-directed basis, using full, rather than reduced, constellation comparisons.
While the incorporation of reduced constellation slicing proposed by the Jablon publications into a conventional decision-directed carrier recovery system, such as that described above, appears to achieve a phase lock in more instances than use of full slicing alone and is thus quite robust, I have discerned that the resulting system possesses various drawbacks which adversely limit its performance particularly with HDTV signal demodulation.
First, conventional decision-directed carrier recovery systems fail to account for variations in carrier-to-noise (CNR) ratio. I have found that these variations--which can occur often, if sufficiently large, can cause a false lock to occur. Specifically, if an (RC) acquisition/(full slicing) tracking strategy were simply incorporated into such a system as, e.g., taught by Jablon, then, a sufficiently large CNR variation would likely cause the system to erroneously remain in a (full slicing) tracking mode when the system should however switch back to an (RC) acquisition mode of operation, thereby causing a false lock to occur. This, in turn, would cause erroneous symbol detection. Second, the RC approach performs phase comparisons on a fixed number of symbols regardless of whether a phase lock is achieved or not during the corresponding time period. I have found that, in certain instances this time period may be excessive, i.e. a phase lock could be achieved in a shorter period of time, while insufficient in others, i.e. a phase lock could not be attained in the time allotted but could be achieved during a longer period.
Thus, a need exists in the art for a carrier recovery system, including apparatus and accompanying methods for use therein, which, when used in a QAM demodulator, advantageously and substantially eliminates false locks and also achieves a phase lock in far more instances than occur in the art. Furthermore, this carrier recovery system should acquire a phase lock over fewer symbols and hence faster than has occurred through conventional carrier recovery systems. In addition, this system should also provide an accurate phase lock over a wide range of CNRs. Advantageously, such a carrier recovery system should find wide use in a HDTV demodulator and, in those instances where repeated re-acquisition of the carrier phase lock is likely to occur, will result in less lost data and hence, e.g., a more pleasing image than that which would otherwise result from using such a conventional carrier recovery system.